Bubbles generation device and method

ABSTRACT

There is disclosed a device ( 1 ) for generating fine bubbles, comprising a substrate ( 3 ) having holes ( 5 ) therethrough, each hole comprising a gas inlet ( 7 ) and a gas outlet ( 9 ), wherein the width of the gas outlet is greater than the width of the gas inlet. A method of manufacturing said device and a method of generating fine bubbles.

The present invention generally relates to a device for generatingbubbles and, more particularly, to a device for generating fine bubblesand various uses thereof. The invention also comprehends a method ofmanufacturing said device and a method of generating fine bubbles.

The formation of fine bubbles is a well studied and reported area ofresearch. Recently, attention has been paid to various methods ofutilizing fine bubbles having a diameter of micrometer level, andvarious apparatus for generating fine bubbles have been proposed.

Some of the common techniques used to form gas bubbles include:compressed air to dissolve air into a liquid stream, which is thenreleased through nozzles to form bubbles by cavitation; air streamsdelivered under a liquid surface, where bubbles are broken offmechanically, say by agitation or shear forces; and ultrasonic inducedcavitation.

In a system to generate air bubbles by introducing air into water flowwith a shearing force using vanes and an air bubble jet stream, it isoften required to employ a higher number of revolutions to generatecavitation. However, problems arise such as power consumption increaseand corrosion of vanes or vibration caused by the generation ofcavitation. Further, such a technique does not lend itself to generatinglarge amounts of fine bubbles.

The desire for small bubbles is that they provide a variety of excellenteffects, which have been utilized in many industrial fields includingplant cultivation, aquafarming, wastewater treatment and the like. It iseffective to reduce the diameter of bubbles to increase their surfacearea relative to their volumes, thereby enlarging the contact areabetween the bubbles and the surrounding liquid; thus, a more rapid masstransfer process can take place when the bubble size is reduced.

In wastewater treatment plants, it is known to aerate effluent, orsludge, as part of the wastewater purification process. Generally, airis introduced near the bottom of an aeration tank containing wastewaterand bacterial floc via a system of pipes and/or hoses. As the air risesto the surface as air bubbles, some of the oxygen in the air istransferred to the wastewater and is consumed by the respiring bacteriaduring digestion which aids in the treatment of sewage. The more oxygenthat is supplied to the bacteria, the more efficient the digestionprocess. It is desirable, therefore, to provide smaller bubbles wherebyto enhance further the efficiency of the digestion process.

A similar requirement exists in bioreactors and fermenters in caseswhere they are sparged for aeration purposes. Specifically, the yeastmanufacturing industry has the requirement where growing and reproducingyeast bacteria need constant oxygen replenishment for respirationpurposes.

However, in an aeration system using a conventional-type fine bubblegenerating system, for instance a diffusion system based on injection,even when fine pores are provided, when air bubbles are injected underpressure through pores, the volume of each bubble is expanded and thediameter of each bubble is increased to several millimetres due to thesurface tension of the air bubbles during injection. Such a methodencounters difficulty in generating fine bubbles of small diameter.Another problem associated with such a method is the clogging of thepores, which reduces the efficiency of the system.

A further application of small bubbles is the extraction of hard-to-liftoil reserves in some fields which either have little oil left, or havethe oil locked in sand. Bubbling gas up through such oil-bearingreserves has the effect of lifting the oil as the bubbles rise undergravity and bringing the oil with them. The bubbles are formed in waterand pumped into the well or reserve and the oil is carried at theinterface between the gas and water of each bubble as it passes throughthe reserves. Hence, the smaller the bubble, the greater the relativesurface area for transport of the oil.

It is thus desirable to generate fine bubbles in a more convenient andefficient manner than known hitherto.

The general perception is that in order to reduce the size of a bubble,the solitary requirement is for the pore size through which the bubbleis formed be reduced. However, there are a number of reasons why thisperception is ill-conceived.

The first of these reasons is that the bubble is “anchored” to thesubstrate material through which it is formed, and will continue toinflate until the bubble breaks free by some disruptive force. Theforces can, for instance, be buoyancy, inertial or shear forces appliedto the bubble as it develops. The interfacial tension controls the forcewith which the bubble is held by virtue of it being anchored to thesurface. In this way, there are three interactions that need beconsidered:

-   -   the interaction between the liquid and solid substrate, γ_(ls),        [mN/m];    -   the interaction between the liquid and gas (i.e air), γ_(lg),        [mN/m]; and    -   the interaction between the solid substrate and gas (i.e air),        γ_(sg), [mN/m].

The relative contribution of these forces controls the nature of bubblegrowth and the ease with which the bubble is able to break away from thesurface.

In addition to the above, the rate of bubble growth,

$\frac{r}{t},$

is independent of the hole size, but can be expressed as:

$\begin{matrix}{\frac{r_{b}}{t} = \frac{F}{4\pi \; r_{b}^{2}}} & {{Formula}\mspace{14mu} (1)}\end{matrix}$

Where F is the flow rate of gas going through the hole, and r_(b) is theradius of the bubble. This implies that the smaller the bubble, thefaster the rate of growth, and this can clearly be seen from FIG. 1which shows the rapid growth of a bubble through a single hole of 30microns diameter.

It has been observed that bubbles form more or less instantaneously atthe surface of a pierced or sintered material. As a gas bubble emergesfrom a hole into a liquid, the shape of the bubble is assumed to be aspherical cap of radius (r), and height (h), as seen in FIG. 1. Thevolume of the bubble is thus given by:

$\begin{matrix}{V = {\frac{\pi}{3}{h^{2}\left( {{3r} - h} \right)}}} & {{Formula}\mspace{14mu} (2)}\end{matrix}$

There exists a geometrical relationship between bisecting cords thatallows Formula (2) to be transformed to a single variable, h, such that:

$\begin{matrix}{V = {\pi\left( {\frac{r_{o}h^{2}}{2} - \frac{h^{3}}{6}} \right)}} & {{Formula}\mspace{14mu} (3)}\end{matrix}$

The rate of volumetric growth of a bubble, dV/dt, is equal to the flowrate of gas, F, through the hole, given a constant differentialpressure, ΔP=P₀−P₁. The differential identity:

$\begin{matrix}{\frac{V}{t} = {\frac{h}{t} \cdot \frac{V}{h}}} & {{Formula}\mspace{14mu} (4)}\end{matrix}$

Formula (4) can be used to give the linear rate of growth of the bubble:

$\begin{matrix}{\frac{h}{t} = \frac{2F}{\pi \left( {r_{o}^{2} + h^{2}} \right)}} & {{Formula}\mspace{14mu} (5)}\end{matrix}$

A numerical solution of Formula (5) gives the rate of growth of thebubble radius, as seen in see FIG. 2. The analysis shown in FIG. 2 isconsistent with the observation that bubbles form more or lessinstantaneously at the surface of a pierced or sintered material. Theextremely rapid growth of the bubble radius from the initial conditionof r₀=15 μm to a radius of 250 μm (0.5 mm diameter) in 0.01 seconds canbe seen. This is followed by a relatively steady growth rate. Thesolution of Formula (5) is thus also consistent with the observations.

It is known, from the Young-Laplace equation, that the maximum pressure,ΔP, within the bubble is achieved when the bubble is at its smallestradius, r_(b):

$\begin{matrix}{{\Delta \; P} = {{P_{{inside}\mspace{14mu} {bubble}} - P_{{outside}\mspace{14mu} {bubble}}} = \frac{2\gamma}{r_{b}}}} & {{Formula}\mspace{14mu} (6)}\end{matrix}$

Where γ is the liquid/gas interfacial tension.

The minimum radius of the bubble occurs when the bubble is a hemisphereof the same radius of the hole through which it passes, which thereforeconstitutes the point of maximum pressure in the bubble; this is knownas the break-through pressure.

It is desirable to provide means and a method for removing bubbles fromthe surface of the substrate and into the liquid before the bubble growstoo large. It may be desirable, for instance, to generate fine bubbleshaving diameters of 100 microns or less, and preferably 50 microns orless.

From the description that is to follow, it will become apparent how thepresent invention addresses the deficiencies associated with knowntechniques and provide numerous additional advantages not hithertocontemplated or possible with known constructions.

The inventors postulated that if the flow of air, F, could be stopped onreaching the break-through pressure of the bubble, then the residualpressure in the hole would inflate the bubble to a small size, whichsubsequently could be cleaved from the surface, before flow was resumed.

A reservoir of air would be formed in the pore that feeds the bubble,which is of depth δ, and radius r_(o.) From Boyle's law, the initialstate and bubble break-through, subscript “0”, can be related to thefinal state, subscript “1”:

p_(o)V_(o)=p₁V₁   Formula (7)

The volumes at the initial and final states are given by:

$\begin{matrix}{{V_{o} = {{2\pi_{o}^{2}\delta} + {\frac{2}{3}\pi \; r_{o}^{3}}}}{V_{1} = {{2\pi \; r_{o}^{2}\delta} + {\frac{4}{3}\pi \; r_{1}^{3}}}}} & {{Formula}\mspace{14mu} (8)}\end{matrix}$

The potential bubble size can be determined by combining Formulas (6),(7) and (8), and simplifying gives the relationship in the form ofdepressed cubic expression:

2r ₁ ³−(3δr _(o) +r _(o) ²)r ₁+3δr _(o) ²=0   Formula (9)

Where δ is the length/depth of the hole (i.e thickness of substrate),r_(o) is the radius of the hole through which the bubble is formed, andr_(b) is the final bubble size.

The solutions to Formula (9) are clearly dependent on the values of r₀and δ. For typical values of r₀=15 μm and δ=70 μm, the solution toFormula (9) gives, r₀=30 μm, a bubble twice the size of the hole, butwithin the desired range. Table 1 gives a range of solutions for which abubble will form when equilibrium is reached. These final values are allof a size which would provide good mass transfer if formed in largequantities.

TABLE 1 Hole Plate thickness radius 80 70 60 50 40 [μm] [μm] [μm] [μm][μm] [μm] 5 22 20 19 16 14 10 29 27 24 21 15 33 30 26 20 35 30 23 35

The inventors considered means for stopping the gas flow at the point ofbreak-through of the bubble. As there are tens of thousands of holes ina typical porous element, individually stopping flow in each hole wouldseem prohibitively complex.

A number of alternative routes were considered which included:

-   -   oscillatory or pulsed gas flow;    -   using a sonic frequency actuator, such as a speaker, to cause        pressure peaks and troughs;    -   a flexible membrane that would open and close the rear of the        holes as gas flowed; and    -   use of orifice plates to restrict flow during the expansion        phase of growth.

Thus, according to a first aspect of the present invention, there isprovided a device for generating fine bubbles, comprising a substratehaving holes therethrough, each hole comprising a gas inlet and a gasoutlet, wherein the width of the gas outlet is greater than the width ofthe gas inlet; wherein the average gas inlet width ranges from about 2to 10 microns; and/or wherein the average gas outlet width ranges fromabout 5 to 100 microns; and/or wherein the inlet diameter is 1/10^(th)to ⅕^(th) of the outlet diameter.

It was found that use of an orifice having an outlet size greater thanthe inlet size for the purpose of restricting flow would deliver thesought-after properties. As the bubble expands beyond the break-throughpressure, air supply would be limited by the restriction of the orifice,and so the flow term, F, in Formula (1) is reduced and consequently therate of growth of the bubble is decelerated. In this way, there isprovided a way in which to minimise the bubble size while retaining thecapability of generating large amounts of fine bubbles. The complexityof the device is far less than that of known devices, such as those thatemploy cativation as the predominant technique for generating finebubbles.

With regard to the device, the cross-sectional shape of each hole may beone selected from circular, triangular, square, rectangular, pentagonal,hexagonal, heptagonal, octagonal, nonagonal and decagonal. Of course, itwill be appreciated that other geometric shapes may be equally aseffective in achieving the function of the present invention. A circularcross-section may in some embodiments be particularly effective ingenerating fine bubbles.

The device may be a porous member or may be regarded as such. The porousmember may be utilised in applications such as redox fuel cells,particularly regeneration systems for such cells.

The width of the gas outlet may be an order of magnitude greater thanthe width of the gas inlet. Typically, the outlet diameter may be lessthat half the diameter of the desired bubble diameter. The inletdiameter should be small enough to choke the gas flow. This effect isparticularly seen when the inlet diameter is 1/10^(th)(0.1) to⅕^(th)(0.2) of the outlet hole diameter. Gas Inlet widths in the rangeof 2 microns to 10 microns and gas outlet widths in the range from 20microns to 50 microns in diameter are particularly effective.Consistency on geometry across the perforated surface may be critical tothe effective performance of the devices; thus, a variation of no morethan 10% is preferred; more particularly a variation of less that 1% maybe desirable.

The hole may taper regularly from the gas inlet towards the gas outlet.Depending on the liquid-solid interfacial tension, the shape of the holemay prevent ingress of liquid into the holes. For example, a hydrophobicsubstrate with an open conical structure may desirably prevent liquidingress.

The hole may taper irregularly from the gas inlet towards the gasoutlet.

The hole density on the substrate may range from 400 to 10000 holes/cm².The hole density is a balance between pressure drop and coalescence ofthe bubbles. The higher frequency of bubble generation, the more likelythey are to coalesce when the hole packing density is high. Hole densityof 1,000 to 2,500 holes/cm² may be particularly favourable when using asquare pitch. Stagger pitches have also been used to maximise bubbleseparation. Hexagonal and other pitches have been effective. Of course,it will be appreciated that other geometric pitches may be equally aseffective in achieving the function of the present invention.

The thickness of the substrate may range from 20 to 1,000 microns; thethickness will impact the final bubble diameter as the reservoir of airincreases. The preferred range may be 50 to 100 microns. Of course, itwill be understood that other thicknesses of the substrate may be moresuitable in different applications of the invention.

The gas outlet width may be about half the width of the desired bubblesize, wherein the desired bubble size may range from 50 to 100 microns.

The substrate may have an active surface towards the gas outlets of theholes. The active surface is the surface in contact with the liquidphase in which the bubbles are to be dispersed. Having an active surfacethat attracts the liquid phase, for example a hydrophilic surface in thecase as an aqueous liquid, is advantageous in producing small bubbles asthe liquid favourably flows under the forming bubble and lifts thebubble from the surface, thereby enhancing fine bubble generation.

The wetting of the surface at which the bubbles form can be significant.To this end, at least a portion of the active surface may be formed ofor coated with a hydrophilic material. A hydrophilic surface allows theliquid to “get under” the bubble as it grows, so as to lift it off andthus generate smaller bubbles.

It may be that at least a portion of the surface towards the gas inletof the holes is formed of or coated with a hydrophobic material.Additionally, or alternatively, at least a portion of the interior ofthe holes may be coated with a hydrophobic material. A hydrophobicmaterial does not wet and so the ingress of liquid into the holes oracross to the gas side when the device is not active, under gaspressure, is prevented and allows rapid start-up with minimalbreak-through pressure requirements.

The substrate may be an elongate member. An elongate member may beparticularly suitable for applications such as regeneration of catholytesolutions in redox fuel cells.

The gas outlet may comprise a lip projecting away from the activesurface. The lip may act to lift the exiting-bubble higher in thelaminar boundary layer of liquid flow, and to increase shear stresses todetach the bubble from the solid substrate. Of course, the substrate maybe flexible which may improve its bubble-detachment capabilities.

The device may comprise one or more of sintered glass or metal powders,plastics, porous membranes, meshes and drilled or punctured sheets.

In some embodiments, it may be preferred that the device comprisesstainless steel foils and/or polyimide films. These materials may bereadily formed as thin sheets/substrates, which ability/property lendsitself to the intended function of the invention.

In some embodiments, it may be that the angle of taper from the gasinlet towards the gas outlet is relative to the longitudinal axis ofeach hole and ranges from about 6° to 26°, and preferably from about 10°to 15°. Such angles, combined with the orifice diameter, providesuperior control over the bubble size formation by limiting thereservoir of gas available for bubble formation.

It may be particularly advantageous that each hole is a frusto-conicalshape.

In a second aspect, the present invention comprehends a method ofmanufacturing a device for generating fine bubbles, comprising the stepsof:

-   -   providing a substrate; and    -   perforating the substrate at predetermined locations with holes        of predetermined widths.

Selecting the locations of the holes in the substrate may be significantto prevent coalescence of the bubble at the substrate surface. A welldesigned pattern of distribution will allow bubbles to form and bereleased into the liquid without other bubbles impacting during theformation process. A random distribution of holes may not necessarilyallow this level of engineered control.

The step of perforating the substrate may involve using a laser. A laserprovides an accurate way in which to perforate the substrate in term oflocation and size of the holes. This may be by way of forming a mastertemplate using laser machining and then mass producing sparge elementsby electroplating or electro deposition.

In a third aspect, the present invention envisages a method ofgenerating fine bubbles, comprising the steps of:

-   -   providing a device according to any of claims 1 to 25 (or as        defined herein);    -   supplying the holes with a liquid; and    -   feeding a gas through the holes via the gas inlet of each hole.

In some embodiments, it may be preferred that the liquid is suppliedacross the holes to induce flow of the liquid. This may significantlyreduce the resultant bubble size. The viscous drag on the forming bubbleprovided by liquid flow is a significantly larger force them buoyancybetween the gas and the liquid. The viscous drag overcomes the adhesionform interfacial tension more rapidly and so smaller bubbles are formed.

While the device for generating bubbles described hereinbefore hasvarious applications, a particularly effective application is use of thedevice in a catholyte regeneration system for a redox fuel cell, forexample.

In an indirect or redox fuel cell, the oxidant (and/or fuel in somecases) is not reacted directly at the electrode but instead reacts withthe reduced form (oxidized form for fuel) of a redox couple to oxidiseit, and this oxidised species is fed to the cathode.

There are a number of constraints on this step of oxidising the redoxcouple. Oxidation of the redox couple should occur as rapidly aspossible as a reduction in flow rate of the catholyte through thecathode will reduce the rate of energy production. The rate of energyproduction will also be reduced if oxidation of the redox couple is notas complete as possible, i.e. if a significant proportion of the redoxcouple remains unoxidised. The provision of apparatus which rapidly andcompletely oxidises redox couples present in catholyte solutions is madechallenging by the need to ensure that the energy consumed when theoxidation step is taken is relatively low, otherwise the overall powergeneration performance of the fuel cell will be reduced. Additionally,the apparatus used to oxidise the redox couple should be as compact aspossible, especially when the fuel cell is intended for use in portableor automotive applications.

The need to balance these conflicting requirements gives rise toinefficiencies in cell performance, particularly in automotiveapplications and in combined heat and power.

The device for generating fine bubbles may be taken to be a porousmember.

In operation of a redox fuel cell, the catholyte may be provided flowingin fluid communication with the cathode through the cathode region ofthe cell. The redox mediator couple is at least partially reduced at thecathode in operation of the cell, and at least partially re-generated byreaction with the oxidant after such reduction at the cathode. The atleast partial regeneration of the redox mediator couple is effected inthe regeneration zone. Specifically, the interfacial area of oxidantpassing through the active surface of the porous member and thecatholyte flowing towards or adjacent to the porous member is large.Regeneration of the redox mediator couple begins at this point andcontinues as the catholyte, with oxidant entrained therein, passesthrough the reoxidation zone.

In a preferred arrangement, at least a portion of the channel wall maybe open to expose the interior of the catholyte channel to at least aportion of the active surface of the porous member.

The porous member may be formed of any porous material that permits thethroughflow of the oxidant in sufficient volumes to enable the at leastpartially reduced redox couple to be at least partially re-generated,i.e. oxidised.

Thus, according to fourth aspect, the present invention contemplates acatholyte regeneration system for a redox fuel cell, comprising: achamber; a first inlet port for receiving into the chamber reduced redoxmediator couple from the cathode region of the cell; a first outlet portfor supplying oxidised redox mediator couple to the cathode region ofthe cell; a second inlet port for receiving a supply of oxidant; and asecond outlet port for venting gas, water vapour and/or heat from thechamber, a catholyte channel in fluid communication with the first inletport, a device according to any of claims 1 to 25 (or as defined herein)having an active surface, and the catholyte channel being arranged todirect a flow of catholyte adjacent to or towards the active surface.

The device may comprise holes having an average diameter of 5 to 100microns, preferably 20 to 50 microns.

By “cathode region” is meant that part of the cell bounded on one sideby the cathode side of the membrane electrode assembly. Alternatively,or as well, the “cathode region” may be thought of as that part of thecell in which at least a part of the catholyte flowing therethrough inoperation of the cell contacts the cathode side of the membraneelectrode assembly.

Likewise, by “anode region” is meant that part of the cell bounded onone side by the anode side of the membrane electrode assembly.

To enhance the performance of the fine-bubble generating device (porousmember), it may be formed or modified specifically to maximise thesurface area of the oxidant passing therethrough. For example, thelocation and size of the pores (holes) may be controlled to encouragethe release of small/fine gas bubbles. Further, the flow ofcatholyte/liquid towards or past the porous member will encourage therelease of small bubbles before they have time to grow. The rapidremoval of bubbles is advantageous as it allows fresh catholyte liquidto contact the active surface of the porous member.

Typically, average bubble size diameters are in the range of 1 to 1000microns. Preferably, the formed bubble size is smaller, for example 150microns in diameter or less, 1 to 100 microns, or most preferably, 25 to50 microns in diameter. To achieve a flow of bubbles having averagediameters falling within these preferred ranges, pores should beprovided having a diameter which is smaller than the target bubblediameter by a factor of 3 to 10 times.

The rapid removal of bubbles can also be encouraged by rendering thesurface of the porous member hydrophilic, either by coating it with ahydrophilic material, or by forming the active surface of the porousmember from a hydrophilic material. The presence of a hydrophilicmaterial on the active surface of the porous member will cause formedbubbles to be more easily released than from a hydrophobic surface.Preferably, such materials will have a surface energy of greater than 46dynes/cm² and/or may include hydrophilic groups, such as hydroxylgroups. An example of such a material is acetate rayon. Additionally, oralternatively, acceptable hydrophilic properties can be achieved bytreating metal surfaces. Such treated metal surfaces include annealedaustentic stainless steel, laser or plasma coated stainless steel oroxide or nitride modified surface coatings.

It will be appreciated that the ingress of catholyte liquid into theporous member may be unfavourable as this will block the throughflow ofoxidant, meaning that the rate of oxidation of that catholyte will bereduced. To overcome this problem, the porous member may be formed of amaterial which is hydrophobic, with a coating on the surface exposed tothe catholyte channel which is hydrophilic. Examples of hydrophobicmaterials from which the porous member may be formed includepolytetrafluoroethylene, halogenated organic polymers, silicone polymersand hydrocarbon polymers such as polythene and polypropylene.Additionally, or alternatively, the maximum pore size can besufficiently small such that the surface tension of the catholyteprevents it from entering the porous member even when no oxidant ispassed through the porous member.

The pores in the porous member, especially its active surface, may beformed using any technique known to those skilled in the art. Inpreferred embodiments, the pores are produced by laser machining orelectroforming.

The catholyte channel and porous member may be arranged in any way,provided that the redox couple in the catholyte solution is at leastpartially regenerated.

In one arrangement, the porous member may be an elongate member. In apreferred embodiment, the porous member is substantially cylindrical ortubular. One or more porous members may be employed in the fuel cell ofthe present invention.

Alternatively, the catholyte channel may be provided as a tube coiledaround the porous member, as a channel co-axial with the porous memberor as an annulus around the porous member. In such arrangements, thechannel wall may be open along substantially all of its length or alongparts of its length to expose the interior of the catholyte channel toparts of the active surface of the porous member. The oxidant issupplied to an interior of the porous member at a positive pressure,causing the oxidant to pass through the outer surface of the porousmember and into the catholyte channel.

In an alternative arrangement, the catholyte channel may be formedwithin the porous member. The catholyte channel may be linear or may behelical. In these arrangements, the active surface of the porous memberis provided on the interior surface of the porous member and oxidant ispassed inwardly through the porous member, through the inner, activesurface and into the catholyte channel.

In certain embodiments, most preferably those where the interior of thecatholyte channel is exposed to the active surface of the porous memberaround the majority, if not the totality of its circumference, theregeneration zone of the fuel cell may preferably comprise catholyterotation means, to maximise the exposure of the catholyte to the activesurface, thus maximising oxidation of the redox couple. The rotationmeans could comprise an offset liquid inlet, to cause helical flow ofthe catholyte through the channel. Additionally, or alternatively, therotation means may comprise a diversion member to induce rotational flowthrough the channel by means of, for example, spiral-like protuberances.

In a further arrangement, the regeneration zone may comprise generallyplanar porous members which define one or more walls of a chamber. Anopen end of the catholyte channel is provided to ensure that the streamof catholyte exiting the catholyte channel is directed toward and flowspast at least a portion of the active surface of the porous member. Thismay be achieved by locating the open end of the catholyte channelsubstantially adjacent to the planar porous member or a slight distanceaway from, but pointed toward the porous member.

The flow rate of the catholyte through the regeneration zone ispreferably relatively high. In a preferred embodiment, the flow rate ofthe catholyte, as it contacts or passes adjacent to the active surfaceof the porous member is at least about 0.2 m/s. In an especiallypreferred embodiment, the flow rate of the catholyte as it contacts orpasses adjacent to the active surface of the porous member is about 0.5to about 1.5 m/s.

The sparging of air into the liquid through the porous member (device)introduces bubbles in the catholyte and may form froth. The fine air(oxidant) bubbles provide an increased surface area that promotestransfer of oxygen and the desired oxidation of the liquidcatalyst/mediator system.

The device may be advantageously employed in the regeneration systembecause smaller contact volume is required for a given amount ofreaction, and so a smaller and more portable regenerator can be built.

According to a fifth aspect, the present invention provides a use of thedevice according to any of claims 1 to 25 in generating fine bubbles.

According to a sixth aspect, the present invention comprehends a use ofthe device according to any of claims 1 to 25 in a catholyteregeneration system of a redox fuel cell.

According to a seventh aspect, the present invention contemplates a useof the device according to any of claims 1 to 25 in treating sewagewater with small oxygen bubbles.

According to a eighth aspect, the present invention envisages a use ofthe device according to any of claims 1 to 25 in growing and reproducingyeast bacteria with small oxygen bubbles.

According to a ninth aspect, the present invention provides a use of thedevice according to any of claims 1 to 25 in carbonisation of beverageswith small carbon dioxide bubbles.

According to a tenth aspect, the present invention comprehends a use ofthe device according to any of claims 1 to 25 in removing oil fromoil-bearing reserves such that oil is carried at the interface betweenthe gas and liquid of each small bubble as it bubbles therethrough.

Various embodiments of the present invention will now be moreparticularly described by way of example only, with reference to, and asshown in the accompanying drawings, in which:

FIG. 1: is a schematic diagram of a gas bubble as it emerges from atraditionally-shaped hole (PRIOR ART);

FIG. 2: is a graph of bubble growth for a fixed feed flow (7 μl/s) andhole size of 30 μm diameter;

FIG. 3: is an embodiment of the present invention in the form of aschematic diagram (side view) and descriptive Formulae for bubbleformation through a conical hole;

FIG. 4: is a plan view of a device formed in accordance with anembodiment of the present invention;

FIG. 5: is a plan view of a hole formed in a substrate shown from thelaser entry (gas outlet);

FIG. 6: is a plan view of a hole formed in a substrate shown from thelaser exit (gas inlet);

FIG. 7: is a photograph of fine-bubble formation in static water;

FIG. 8: is a photograph of fine-bubble formation by induced liquid; and

FIG. 9: is a photograph of bubble formation in Polyoxometalate usingcurrent invention.

With reference to FIG. 3, there is shown a schematic diagram (side view)and descriptive Formulae for bubble formation through a conical hole.The inventors found that the use of an orifice to restrict flow by ahole made as a truncated cone would deliver the sought-after propertiesin accordance with the present invention. As the bubble expands beyondthe break-through pressure, air supply would be limited by therestriction of the orifice, and so the flow term, F, in Formula (1) isreduced and the rate of growth is decelerated. A schematicrepresentation of the break-through point is given in FIG. 3, togetherwith the descriptive formulae for the flow rate through the orifice andthe rate of bubble growth, in which:

F=flow rate of gas=rate of change of volume in the bubble=dV/dt

r_(i)=gas inlet hole radius

r_(b)=radius of bubble

r_(o)=gas outlet hole radius

P_(i)=Inlet pressure of gas

P_(b)=bubble internal pressure

ΔP=orifice pressure drop

ΔP_(b)=pressure drop across bubble=internal pressure−external pressure

ΔP_(p)=liquid flow pressure drop

l_(p)=liquid flow characteristic dimension

R=pipe flow radius

d=pipe flow diameter

c_(d)=orifice discharge coefficient

t=time

dU/dy=liquid rate of strain=velocity profile

A_(x)=orifice cross sectional area

ρ=gas density

γ=interfacial tension

τ=viscous shear stress

μ=liquid viscosity

π=3.1415927

To embody this invention into a practical sparge plate (device) theabove design formulae were used to calculate the size of entry and exitholed in the truncated cone and resulting flow rate of air. Typicalvalues are shown in Table 2 below.

TABLE 2 Inner Radius Outer Radius Angle Flow rate [μm] [μm] [°][Litre/min] Number of Holes 10000 2.5 25 18 0.5 10000 1 8 5 0.08 TestExample  286 1.5 10 7 0.01

A test piece to prove the concept was made from 70 μm 316 stainlesssteel foil using a laser drilling technique. As part of the drillingtechnique a lip was formed on the laser entry side, gas outlet side ofthe hole, of 4 μm. The lip around the gas outlet side is of benefit asit lifts the bubble higher in the laminar boundary layer of liquid flow,and increases shear stressed to detach the bubble from the solidsubstrate.

FIG. 3 illustrates a side view of a device for generating fine bubble,generally indicated (1), comprising an elongate substrate (3) having atleast one hole (5) therethrough, the hole (5) comprising a gas inlet (7)and a gas outlet (9), wherein the width of the gas outlet (9) is greaterthan the width of the gas inlet (7).

The hole (5) is conical in structure and has inclined walls (11)extending from the gas inlet (7) to the gas outlet (9). The incline(taper of the hole), in this embodiment, is regular. In the schematicrepresentation of FIG. 3, the walls (11) incline at an angle of 15°relative to the longitudinal axis of the hole (5)

As air passes through the hole (5), a bubble (13) is formed at the gasoutlet (9) side of the hole (5).

With reference to FIG. 4, there is shown a plan view of the device (1)of FIG. 1. The device (1) comprises a substrate (3) formed from 316stainless steel. The substrate (3) comprises 36 holes (5), each of 50microns in size. This diagram is not to scale. The holes (5) arearranged in a 6×6 square pattern.

EXAMPLES

The practical embodiment of this invention was tested using a 10 mm by10 mm section of laser drilled stainless steel plate mounted and sealedin an acrylic block with a regulated air supply. The entry and exitholes of the test piece were examined using SEM—see FIGS. 5 and 6. Thegas entry (inlet) hole was measured at 3.3 μm and the gas exit (outlet)hole was measured at 19.1 μm.

The test plate was mounted horizontally. Air pressure of 250 mbar wasapplied to the feed of the test plate (small holes) and distilled wateradded to cover the surface. Bubble formation was observed andphotographed. Bubbles accumulated and coalesced at the surface. Waterflow across the holes was induced using a wash bottle of distilledwater; again bubble formation was observed and photographed. Much finerbubbles were observed with the induced water flow.

These experiments were repeated using reduced polyoxometalate (0.3Msolution).

From the photographs shown in FIGS. 7 and 8, using relative scaling, thebubbles under induced water flow were estimated to be of the order of 50μm diameter. In FIG. 7, water bubbles are breaking at the surface andmicro droplets of water can be seen above the surface of the water. InFIG. 8, surface bubbles of 1 mm can also be seen.

From the photograph shown in FIG. 9, the measurements with POM showedlarger bubbles, of the order of 75 μm to 100 μm diameter. However, dueto the opaque nature of POM only surface bubbles could be observed, andthese were consistent with those in the water experiment.

1. A device for generating fine bubbles, comprising a substrate havingholes therethrough, each hole comprising a gas inlet and a gas outlet,wherein the width of the gas outlet is greater than the width of the gasinlet; wherein the average gas inlet width ranges from about 2 to 10microns; and/or wherein the average gas outlet width ranges from about 5to 100 microns; and/or wherein the inlet diameter is 1/10th to ⅕th ofthe outlet diameter.
 2. The device of claim 1, wherein thecross-sectional shape of each hole is one selected from circular,triangular, square, rectangular, pentagonal, hexagonal, heptagonal,octagonal, nonagonal and.
 3. The device of claim 1, wherein the deviceis a porous member.
 4. The device of claim 1, wherein the width of thegas outlet is an order of magnitude greater than the width of the gasinlet.
 5. The device of claim 1, wherein the hole tapers regularly fromthe gas inlet towards the gas outlet.
 6. The device of claim 1, whereinthe hole tapers irregularly from the gas inlet towards the gas outlet.7. The device of claim 1, wherein the hole density on the substrateranges from 400 to 10000 holes/cm²
 8. The device of claim 1, wherein thesubstrate has a square pitch and the hole density on the substrateranges from 1000 to 2500 holes/cm².
 9. The device of claim 1, whereinthe thickness of the substrate ranges from about 20 to 1000 microns. 10.The device of claim 1, wherein the thickness of the substrate rangesfrom about 50 to 100 microns.
 11. The device of any of claim 1, whereinthe gas inlet width ranges from about 0.1 to 0.2 gas outlet width. 12.The device of any of claim 1, wherein the average gas outlet widthranges from about 20 to 50 microns.
 13. The device of any of claim 1,wherein the variation in the width of the gas outlet and/or gas inlet isno more than 10%.
 14. The device of any of claim 1, wherein the gasoutlet width is about half the width of the desired bubble size.
 15. Thedevice of claim 1, wherein the substrate has an active surface towardsthe gas outlets of the holes.
 16. The device of claim 15, wherein atleast a portion of the active surface is formed of or coated with ahydrophilic material.
 17. The device of claim 1, wherein at least aportion of the surface towards the gas inlet of the holes is formed ofor coated with a hydrophobic material.
 18. The device of claim 1,wherein at least a portion of the interior of the holes is coated with ahydrophobic material.
 19. The device of claim 1, wherein the substrateis an elongate member.
 20. The device of claim 15, wherein the gasoutlet comprises a lip projecting away from the active surface.
 21. Thedevice of claim 1, comprising one or more of sintered glass or metalpowders, plastics, porous membranes, meshes and drilled or puncturedsheets.
 22. The device of claim 21, comprising stainless steel foiland/or polyimide films.
 23. The device of claim 1, wherein thecross-sectional shape of each hole is circular, and the widths of thegas inlet and gas outlet are the diameters of the gas inlet and gasoutlet.
 24. The device of claim 23, wherein the angle of taper from thegas inlet towards the gas outlet is relative to the longitudinal axis ofeach hole and ranges from about 6° to 26°.
 25. The device of claim 23,wherein each hole is a frusto-conical shape.
 26. A method ofmanufacturing a device for generating fine bubbles, comprising the stepsof: providing a substrate; and perforating the substrate atpredetermined locations with holes of predetermined widths, optionallywherein the step of perforating the substrate involves using a laser.27. (canceled)
 28. A method of generating fine bubbles, comprising thesteps of: providing a device according to claim 1; supplying the holeswith a liquid; and feeding a gas through the holes via the gas inlet ofeach hole, optionally wherein the liquid is supplied across the holes toinduce flow of the liquid. 29-50. (canceled)